1. Field of the Invention
The present invention relates to a magnetic toner used for recording methods utilizing electrophotographic methods and the like.
2. Description of the Related Art
A large number of methods are known as the electrophotographic method. Generally speaking, in the method, an electrostatic latent image is formed on an electrostatic latent image bearing member (hereinafter, also referred to as a “photosensitive member”) utilizing photoconductive materials with the aid of various techniques. Successively, the latent image is rendered visible by developing it with a toner. The thus formed toner image is transferred to a recording medium such as paper where necessary and is then fixed on the recording medium by the application of heat or pressure to produce a duplicate. Examples of such an image forming apparatus include a copying machine and a printer.
Such printers and copying machines recently undergo the progress of the transition from analog to digital apparatuses, and are intensely required to be excellent in the reproducibility of the latent image and high in resolution, and at the same time to always offer an image of high image quality in a stable manner even under various use circumstances. The various use circumstances as referred to herein mean the use conditions as well as the installation environment and the operation environment of printers and the like.
From the viewpoint of the ways of use of the printers, medium- or high-speed printers operated in offices or the like are large in print volume and high in operation rate, and on the contrary, compact, low-speed printers are small in print volume and sometimes are left unused for printing over a long period of time.
It has been realized that as a result of the printers being left unused for a long period of time, specific problems ascribable thereto occur. Specifically, there occurs a problem of image density degradation after a long-term retention of printers in an environment of high temperature and high humidity. Such a problem tends to conspicuously occur particularly in a case where printers have been left unused for a long period of time after attainment of the conditions that the amount of the remaining toner becomes small due to printing of a large number of sheets with a low coverage rate and a small number of printed sheets per one job. This is ascribable to the reason that the low coverage rate of each printed sheet enables printing of a large number of sheets to thereby accelerate the degradation of the toner, or alternatively, the low coverage rate results in exclusively selective consumption (what is called “selective development”) of the toner particles retaining an appropriate amount of charges and hence the fraction of the toner particles retaining an appropriate amount of charges is gradually decreased to cause difficulty in performing a desired development.
After printing of a large number of sheets, the chargeability of the toner is degraded, and consequently, the shading unevenness called “ghost” tends to occur on the image.
When printers are left unused in an environment of high temperature and high humidity, the toner eventually absorbs water to disturb the charging, and hence the developability may be degraded. The water absorbability of the toner mainly depends on the raw materials constituting the toner and the state of being of the toner. In general, the magnetic material used in a magnetic toner is more hydrophilic and more easily absorbs moisture as compared with the binder resin. On the other hand, toners obtained by pulverization (hereinafter, referred to as pulverized toners) tends to undergo the exposure of the magnetic material on the toner surface and tends to absorb moisture.
In this connection, there have been proposed toners improved in the environmental stability by making a magnetic material contain silicon and by controlling the state of being of the magnetic material (see Japanese Patent Application Laid-Open Nos. H05-72801 and H11-316474). However, even the use of such toners has left room for improvement of the density stability and ghost when allowed to stand after continuous running in an environment of high temperature and high humidity.
Further, there has been offered a proposal that the environmental stability is improved by specifying the content of silicon in the magnetic material, and, at the same time, by using a magnetic material having been treated with a surface modifying agent to modify the surface (see Japanese Patent Application Laid-Open No. H10-239897). This toner is improved in the environmental stability by enclosing the magnetic material inside the toner particles through performing suspension polymerization with the aid of the thus treated magnetic material and to thereby prevent the exposure of the magnetic material to the surface of the toner particles. However, even the use of such a treated magnetic material has left room for improvement of the density stability when allowed to stand after continuous running in an environment of high temperature and high humidity. This is ascribable to the fact that the magnetic material present in the vicinity of the surface of the toner particles is made to adsorb moisture by being allowed to stand over a long period of time.
As described above, there has been left room for further improvement with respect to the running stability in an environment of high temperature and high humidity and the density stability and ghost when allowed to stand after continuous running.